This invention relates to lubricating oil compositions which are particularly useful as coolants, hydraulic fluids, turbine oils, spindle oils and the like.
The trend today in industry is toward increasing power applications and the use of increasingly sophisticated machine systems such as numeric control machines operating to even closer tolerances to perform a variety of functions. Lubricating functional fluids often become quite hot during operation of many of the systems in which they are employed. These fluids must therefore have long term thermal stability and antioxidation properties in order for the fluids to have reasonably long useful life. Additionally, for many hydraulic systems; the fluids must have good antiwear properties and low tendency toward corrosion. In many applications the functional fluids come in contact with water, either by design, through leakage of seals or worn parts, and through condensation of moisture into fluid reservoirs during periods in which the systems containing them are inoperative. Water contact is of particular concern in functional fluids containing zinc dihydrocarbyl dithiophosphate, which is susceptible to problems of hydrolytic instability, as described, e.g., in U.S. Pat. No. 3,843,542. In addition the water often is incompatible with the compounded fluids at temperatures less than about 100.degree. F resulting in the formation of insolubles and plugging of screens of filters normally present in the mechanical system. These filters or screens are necessary to prevent passage of, e.g., dirt metal chips and other solid particles which could damage or destroy working parts within the system having very close tolerances.
There exists a need for an improved lubricating oil composition which exhibits not only good properties of long term thermal stability, oxidation stability and low wear, but also has good water compatibility and filtration properties as well.